Steep anomalous dispersion in coherently prepared Rb vapor

Steep dispersion of opposite signs in driven degenerate two-level atomic transitions have been predicted and observed on the ${D}_{2}$ line of ${}^{87}\mathrm{Rb}$ in an optically thin vapor cell. The intensity dependence of the anomalous dispersion has been studied. The maximum observed value of anomalous dispersion ( $\mathrm{dn}/d\ensuremath{\nu}\ensuremath{\simeq}\ensuremath{-}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}{\mathrm{Hz}}^{\ensuremath{-}1}$) corresponds to a negative group velocity ${V}_{g}\ensuremath{\simeq}\ensuremath{-}c/23000$.